Systemd hangs when attempting shutdown

It indeed looked like this bug which affects the initial release of systemd 242: https://bugs.gentoo.org/685002 (upstream: https://github.com/systemd/systemd/issues/12335).

And, in fact, the change from 242 to 242r1 (in gentoo's versioning) was to apply this patch: https://gitweb.gentoo.org/repo/gentoo.git/commit/sys-apps/systemd?id=eb1d80e6a30d09f9f139877c5b754c8a8e918d7a